Machines such as, for example, wheel loaders, dozers, backhoes, dump trucks, and other heavy equipment are used to perform many tasks. To effectively perform these tasks, the machines require an engine that provides significant torque through a transmission to one or more ground engaging devices. Often, these machines utilize continuously variable transmissions (CVT) for increased engine efficiency.
A CVT is an automatic type of transmission that provides an infinite number of output ratios within its ratio range. For example, a hydraulic CVT includes a pump and a fluid motor that receives pressurized fluid from the pump. Depending on a discharge flow rate and pressure of the pump, the motor speed and output torque at the ground engaging device may be varied. An electric CVT includes a generator and an electric motor that receives current from the generator. Depending on the current supplied to the motor, the motor speed and output torque may be varied.
An example of a system used to control a conventional CVT is described in U.S. Pat. No. 4,793,217 (the '217 patent) issued to Morisawa et al. on Dec. 27, 1988. The control system disclosed in the '217 patent is a speed based system that adjusts a speed of an input shaft of the CVT to maintain a target output. The CVT operates in a plurality of modes such as forward and reverse, wherein each operation mode has a unique map assigned to it. In addition, each map indicates a relationship between a target speed of the input shaft of the CVT and an engine output for the associated mode. When the CVT is actuated, a controller determines in which mode the CVT is operating and selects the map designed for that mode. The controller then adjusts the speed of the input shaft according to the map to attain the desired engine output.
Although the control system disclosed in the '217 patent may produce a desired engine output for a plurality of operating modes, the application of the system may be limited. In particular, because each input invokes only one map, there may be little variety in the types of maps available. Such a reduced variety may limit the diversity of tasks and environments in which the transmission and ultimately the associated machine may operate.
The disclosed apparatus is directed to overcoming one or more of the problems set forth above.